Contour feathers

The outer contour or vane feathers have a classic feather shape a shaft with flat opposing vanes (see Figure 6.8). There are two classes of contour feathers, the symmetrical body feathers, and the asymmetrical flight feathers that fill out the wings and tail. 

The vanes consist of individual, parallel branches called barbs. The barbs each look like a tiny feather, with two rows of tiny barbed branches called bar-bules. The barbules on adjacent barbs interlock, which gives the feather vane its flexible strength. In non-flying birds like the emu and ostrich, the barbules are not hooked together, resulting in contour feathers that appear fluffy, rather than flat and sleek. 

Some contour feathers include an additional structure, a smaller rachis that emerges from the shaft below the vanes. This afterfeather or aftershaft has branching barbs, but no barbules, so it is downy or fluffy. This type of feather is seen most often in the family Phasianidae (pheasants and grouse). 

Semi-plumes have characteristics of both down and contour feathers. They tend to be short, with some downy barbs. They lie directly beneath the contour feathers, giving both support and insulation. 

Down feathers consist of many very thin, fine branches on a narrow rachis. They are the insulation under the larger, stronger contour feathers, and are the only feathers present on very young birds. 

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